Damage to the product or packaging material being handled is often a problem when clamp-handling cartons or appliances. The trend is to reduce the cost of packaging material by reducing its thickness, stiffness or abrasion resistance. Consequently, current product packaging is often not as rugged as it has been in the past.
Also, lift truck drivers will often over-clamp a load, causing product or packaging damage. In existing systems, typically multiple-position relief valves are used by the lift truck driver to select different clamping forces for various loads. These valves are controlled by the lift truck driver and can be used incorrectly, either by over-clamping the load, or under-clamping the load.
Also, if the truck control valve is operated several times consecutively while revving the truck engine, the clamping force realized is higher than the intended clamping force, which may also damage the load or its packaging.
An additional source of problems with current hydraulic clamping systems is the slight internal hydraulic leakage that can occur within such systems. While transporting a load, slight internal hydraulic leakage in the system can reduce clamping pressure. This may result in an unintended dropping of the load, or, in response to load slippage, the driver may over-clamp to stop such slippage. In either case, damage to the load may occur.
Many current hydraulic force control systems for lift truck load clamping attachments use computers, proportional valves, pressure transducers, related devices, and a feedback loop to adjust the clamping force. These systems are typically very complex and expensive, and also may not react fast enough to adjust the clamping force to a desired level under variable conditions.